The invention relates to a method for stimulating an oilfield, comprising using a scale inhibitor. In oil recovery and, more importantly, in secondary oil recovery (where a fluid, preferably a water-based fluid, is injected into an oil well in order to displace the crude oil), scale formation may cause blockage of pipework and the pores in the oil-bearing strata, thus reducing or even preventing the flow of oil. Thus, the scale decreases oil recovery yields.
In order to address scale formation, scale-inhibiting polymers are known. Treatment levels up to a few hundred parts per million (ppm) are usually effective. The scale-inhibiting polymer is typically added to the fluid to be treated or may be applied to oil bearing formations by means of “squeeze treatment”. Squeeze treatment involves pumping scale inhibitor into an oil production well so that the inhibitor enters the formation rock and is retained there. When the well is put back into production, the produced fluids are treated by the scale inhibitor which leaches out of the formation rock. Thus the scale inhibitor is released in a fluid. The scale inhibitor may be applied in an aqueous or non-aqueous medium. The objective is to prevent or control the scale formation in systems wherein the fluid is used.
Scale formation is only controlled if the scale inhibitor polymer is present at a treatment level within the product's defined effective range, for example of the minimum inhibitor concentration (MIC) of the specific system (water+scale inhibitor). During production, when the inhibitor has been released, for example by consumption, there is a need for re-squeezing. With squeeze treatment, the concentration of the scale inhibitor in the produced fluids will diminish over time till a repeat “re-squeeze” operation is necessary. Also, scale inhibitor may be lost through, e.g. adsorption or degradation. Hence, there is a need to replenish the scale inhibitor to replace this loss. The consequences of scaling is often catastrophic in this application and so it is most important to avoid scale. Overall, it can be seen that the concentration of scale inhibitor in the treated fluids is vitally important and chemical analysis of scale control polymers has always been difficult at ppm levels.
The problem of analysis has recently become more difficult in subsea oilfields because of subsea completions where several individual wells are commoned on the seabed and the combined fluids are piped to the nearest production platform which may be several tens of miles away. In that configuration, if the oil yield decreases in the recovered combined fluid, it is not possible to determine the particular well that has too much scale, and/or to determine the well wherein scale inhibitor should be added. Because of that it is sometimes necessary to stop the production for all the wells, or to add too much scale inhibitor (for example by adding to much scale inhibitor in a well wherein less is needed). That decreases the global productivity and/or is not cost effective.